Previous studies provide significant experimental evidence for the role of pituitary adenylate cyclase-activating polypeptide (PACAP)-mediated signaling in regulation of anxiety in both experimental animals and human subjects. It has been demonstrated that PACAP may regulate anxiety-related behavior through its actions in two interacting brain regions, the amygdala and BNST. However, synaptic and network mechanisms of PACAP-mediated effects in the brain are poorly understood. Here, we propose a combined behavioral and electrophysiological study in order to address specific questions about the nature of PACAP-induced synaptic and network-level modifications in BLA-BNST circuits, possibly contributing to control of anxiety states. In Aim 1, we will explore the role of PACAP in regulation of synaptic and neuronal functions in projections from the BLA to two BNST subdivisions, ovBNST and adBNST, known to regulate anxiety in opposite directions- activation of ovBNST was shown to induce anxiety, whereas activation of adBNST is anxiolytic. We will express the photosensitive protein, channelrhodopsin-2 (ChR2), under control of the neuron-specific promoter CaMKII? in BLA neurons and photostimulate corresponding fibers synapsing on neurons in both BNST subdivisions. As our preliminary findings indicate that PACAP is selectively expressed in ovBNST and potentiates excitatory synaptic responses at inputs to ovBNST but not at inputs to adBNST, we hypothesize that neuropeptide PACAP may contribute to regulation of anxiety states by differentially affecting synaptic efficacy at BLA projections to different BNST subdivisions, and, therefore, modifying the signal flow in BLA- ovBNST-adBNST circuits in such a way that adBNST is inhibited. This would explain the ability of PACAP in BNST to trigger anxiety, as direct optogenetic inhibition of adBNST was shown to be anxiogenic. In Aim 2, we will test a hypothesis that the parabrachial nucleus (PBn) and paraventricular nucleus of the hypothalamus (PVN) might be the sources of endogenous PACAP in BNST, contributing to regulation of anxiety, as both PBn and PVN possess PACAPergic neurons and project to BNST. Thus, using in vivo optogenetics, we will explore the possibility that PBn-BNST and PVN-BNST projections play a functional role in regulation of anxiety states, possibly through the release of PACAP in ovBNST under anxiogenic conditions. With these considerations as a foundation, our overarching goal is to define synaptic and neural network mechanisms which may contribute to regulation of anxiety at the level of interacting components of corresponding brain circuits. These questions are important because anxiety disorders may reflect dysregulation of communication between components of the neural circuits controlling innate fear responses, including the amygdala and BNST.